Topic Contents

Carbon Monoxide (CO)

Test Overview

A carbon monoxide blood test is used to detect
poisoning from breathing carbon monoxide (CO), a
colorless, odorless, poisonous gas. The test measures the amount of hemoglobin
that has bonded with carbon monoxide. This amount is also called the
carboxyhemoglobin level. See a picture of
what happens during carbon monoxide poisoning.

When a person inhales carbon monoxide,
it combines with the red blood cells that normally carry oxygen to the body's
tissues and replaces the oxygen that is normally carried in the blood. As a
result, less oxygen is carried to the brain and other body tissues. Carbon
monoxide can cause severe poisoning and death.

Carbon monoxide is
made during burning when there is not enough oxygen present for complete
combustion. The main sources of carbon monoxide are engine fumes (such as from cars or boats),
fires burning with poor ventilation (such as gas heaters and indoor cooking
fires), factories, and smoking tobacco.

Why It Is Done

A carbon monoxide blood test is used to
detect poisoning from breathing carbon monoxide. You might have this test if
you have been exposed to the gas or if you have unexplained symptoms, such
as:

Headache, dizziness, or vision
problems.

Nausea or vomiting.

Muscle
weakness.

Confusion or trouble thinking.

Extreme
sleepiness.

How To Prepare

Do not smoke before you have this
test.

Talk to your doctor about any concerns you have regarding
the need for the test, its risks, how it will be done, or what the results may
mean. To help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing blood
will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site with
alcohol.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Put a gauze pad or cotton ball over the needle site as
the needle is removed.

Put pressure on the site and then put on a
bandage.

How It Feels

The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
pinch.

Risks

There is very little chance of a
problem from having blood sample taken from a vein.

You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
minutes.

In rare cases, the vein may become swollen after the blood
sample is taken. This problem is called phlebitis. A warm compress can be used
several times a day to treat this.

Ongoing bleeding can be a
problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and
other blood-thinning medicines can make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your doctor before your blood sample is taken.

Results

A carbon monoxide blood test is used to
detect
poisoning from breathing carbon monoxide (CO), a
colorless, odorless, poisonous gas. The test measures the amount of hemoglobin
that has bonded with carbon monoxide. This is also called the
carboxyhemoglobin level.

Carbon monoxide results are reported as
a percentage: The amount of carbon monoxide bound to hemoglobin is divided by
the total amount of hemoglobin (and then multiplied by 100). The higher the
percentage, the greater the risk of having symptoms of carbon monoxide
poisoning. With values below 10%, a person may not have any symptoms of
poisoning.

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Women and children may have more severe symptoms of carbon
monoxide poisoning at lower carbon monoxide levels than men because women and
children usually have fewer red blood cells.

What Affects the Test

Smokers already have some carbon monoxide in their blood.

People
who are regularly exposed to car exhaust, such as taxi drivers and traffic
police, often have high carbon monoxide levels (8% to 12%).

What To Think About

A person with symptoms and possible exposure to carbon monoxide,
such as someone who lives in a house with an old heating system and complains
of ongoing headaches, should be tested for carbon monoxide
poisoning.

A person who may have carbon monoxide poisoning should
be removed from the place of likely exposure and given oxygen to breathe before
being tested.

If
carbon monoxide poisoning is suspected, other tests (such as arterial blood
gases and a complete blood count) may be done. An arterial blood gas (ABG) test
may be done to determine whether symptoms are caused by carbon monoxide
poisoning or by another disease that causes similar symptoms. See the topics
Arterial Blood Gases (ABG) and
Complete Blood Count (CBC).

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